Forward email notifications to signal using signal-cli

This project was built to solve my my use case of sending Grafana alerts via Signal (see grafana/grafana#14952 for the open issue).

Currently this just means that a Docker container running a Python app listens on port 8025 for incoming emails, extracts the subject and the first image (if it exists), and finally utilizes another container running signal-cli (bbernhard/signal-cli-rest-api) to send a message to a signal number set in the receiver address.

If the receiver address does not follow the format as specified below, the email is instead forwarded using an external SMTP server.

Setup

DANGER

If you run this service with port 8025 exposed on a server directly connected to the internet, you will be running an open SMTP relay and bad actors will use it to send spam emails.

If you only send emails from other docker containers, remove the commented out ports: section.

To send messages from local, non-dockerized services, uncomment the lines but make sure the 127.0.0.1 remains so the container is only available via localhost.

Otherwise run the server in a network behind a firewall if you don't want your server to be abused and your IP to be blacklisted.

The easiest way to set up this project is by using docker-compose with a compose file based on the following template:

version: "2.4"

services:
  signal-rest:
    image: bbernhard/signal-cli-rest-api:latest
    container_name: signal-rest
    restart: always
    volumes:
      - ./signal-rest:/home/.local/share/signal-cli
  
  email2signal:
    image: leoverto/email2signal:latest
    container_name: email2signal
    restart: always
    # ports:
    #   - 127.0.0.1:8025:8025
    environment:
      - SIGNAL_REST_URL=http://signal-rest:8080
      - SENDER_NUMBER
      - SMTP_HOST
      - SMTP_USER
      - SMTP_PASSWORD
    depends_on:
      - signal-rest

signal-cli-rest-api needs to be setup by either copying the data folder from a local signal-cli installation into ./signal-rest or exposing port 8080 on the container temporarily and registering a number via the API.

You need to set the variables listed in the docker-compose file (preferrably by using a .env file). If you don't need to forward emails to an external SMTP server you can set the SMTP_* variables to an empty string.

The receiver address for the emails has to follow the format [SIGNAL_NUMBER]@signal.localdomain, otherwise they will be forwarded to the SMTP server.

Grafana

This has not been tested to work with the "Single email" setting. Make sure "Include image" is enabled if you want to receive snapshots of the alert graph.